1. Field of the Invention
The present invention relates generally to a two-phase brushless motor.
2. Description of the Related Art
FIGS. 1 and 2 are plan views of a stator and a disk-shaped rotor, respectively, of a conventional two-phase brushless motor. In the brushless motor, the stator in FIG. 1 comprises two pairs of coils 1.sub.A1, 1.sub.B1, 1.sub.A2, and 1.sub.B2 for producing two phases of varying magnetic fields (hereinafter denoted phase A and phase B). The rotor shown in FIG. 2 comprises four main magnetic poles which are in opposed relation to the stator and is rotationally mounted with respect thereto. An angle .theta., expressed as an electrical angle between effective portions of each of the coils 1.sub.A1, 1.sub.A2, 1.sub.B1, and 1.sub.B2 which produces phase A or B, is not 180.degree. electrical but instead is 120.degree. electrical.
The pair of coils 1.sub.A1 and 1.sub.A2 for producing phase A, and the pair of coils 1.sub.B1 and 1.sub.B2 for producing phase B may be connected in series or in parallel, but in FIG. 1, the coils are respectively in series connection, and an electric current is alternately supplied at every angle of 180.degree. to the coils 1.sub.A1 and 1.sub.A2 and the coils 1.sub.B1 and 1.sub.B2 with the help of a control circuit shown in FIG. 3.
As shown in FIG. 3, the coils 1.sub.A1 and 1.sub.A2 and the coils 1.sub.B1 and 1.sub.B2 each joined in series connection are connected to collector leads of respective transistors Q.sub.A and Q.sub.B and are also each connected to a power source V.sub.S. A Hall-effect device 4 connected to a power source V.sub.CC detects the rotation angle of the rotor, and makes conductive one of the transistor circuits on the one hand and makes non-conductive the other of transistor circuits on the other hand, or vice versa, so that electric current is alternately supplied to either the coils 1.sub.A1 and 1.sub.A2 or the coils 1.sub.B1 and 1.sub.B2
In the rotor of FIG. 2, two pairs of main magnetic poles 2.sub.N and 2.sub.S are each provided having a width .alpha., and respective pairs of supplemental magnetic poles 3.sub.N and 3.sub.S each of width .beta. are sandwiched between the two pairs of main magnetic poles 2.sub.N and 2.sub.S. For the sake of simplicity, reference symbols have been supplied only on the top portion of FIG. 2, although the symbols also apply to corresponding elements of the lower portion, as well.
Curve a in FIG. 4 shows the relationship between rotational angles and torques of the motor under the condition that only the main magnetic poles 2.sub.N and 2.sub.S function, and the width .alpha. of FIG. 2 is 140.degree. electrical and the width .beta. of FIG. 2 is 40.degree. electrical. Similarly, curve b shows the relationship between torque and rotational angles under the condition that only the supplemental magnetic poles 3.sub.N and 3.sub.S function. Curve a +b illustrated in FIG. 5 shows the torque/angle relationship under the condition that both the main magnetic poles 2.sub.N and 2.sub.S and the supplementary magnetic poles 3.sub.N and 3.sub.S function, curve a+b being obtained by superposing curve b on curve a. According to FIGS. 4 and 5, it is apparent that the sharp drop in torque at every switching point in curve a is compensated by an increase in torque on curve b.
In the above two-phase brushless motor, the magnetic poles are symmetrically arranged, so that the same torque waveforms are simultaneously produced by the coils 1.sub.A1 and 1.sub.A2 and the same may be said of the torque waveforms produced by the coils 1.sub.B1 and 1.sub.B2.
The rotor shown in FIG. 2 has the wide magnetic poles 2.sub.N and 2.sub.S and the extremely narrow supplemental magnetic poles 3.sub.N and 3.sub.S, so that it is difficult to magnetize the rotor, particularly the rotor of a brushless motor used in a video tape recorder or the like, because there is a tendency to make brushless motors for video tape recorders or the like smaller in size. Furthermore, as the hysteresis characteristic disperses due to the existence of the wide magnetic poles and extremely narrow magnetic poles, torque ripples are apt to be generated, which thereby often disperses the torque characteristic of the brushless motor.